multi-ratio rotorcraft drive system and a method of changing gear ratios thereof

ABSTRACT

A multi-ratio rotorcraft drive system and a method of changing gear ratios thereof are disclosed. According to one embodiment, the multi-ratio rotorcraft drive system comprises a rotor system comprising one or more rotors and one or more engines. Each engine of the one or more engines is coupled to the rotor system through a multi-ratio transmission. The multi-ratio transmission comprises an output shaft coupled to the rotor system, an input shaft coupled to a respective engine of the one or more engines, a high speed clutch integrated into a high speed gear train, and a low speed clutch integrated into a low speed gear train. The high speed clutch and the low speed clutch are freewheeling clutches without a friction plate and are capable of disconnecting the output shaft and the input shaft in an overrunning condition when the output shaft spins faster than the input shaft. The multi-ratio transmission shifts between the high speed gear train and the low speed gear train by engaging or disengaging the high speed clutch and modulating speed of the coupled engine.

FIELD

The present disclosure relates to the field of rotorcraft systems, andmore particularly to a multi-ratio rotorcraft drive system and a methodof changing gear ratios thereof.

BACKGROUND

Powered lift rotorcrafts such as helicopters and tiltrotors use liftingforces generated by wings or rotor blades that revolve around a mast. Ina conventional rotorcraft, rotor blades are powered by one or moreengines by way of a transmission, and the speed of the transmissioninput is reduced using one or more fixed ratio reduction stages suchthat the speed of the output powering the rotor is lower than the inputspeed by a fixed ratio. Optimization of rotorcraft performance,including noise, range, and efficiency, can be accomplished by varyingrotor speed.

A typical method of reducing rotor speed in a rotorcraft is to reducethe input speed of a transmission provided by an engine, which directlyreduces the rotor speed by a proportional amount. A general problem inperforming this technique is that reducing the operating speed of theengine may result in a loss in engine efficiency or performance,degrading the net performance improvements possible by reducing therotor RPM. This is mainly because the speed range of the engine thatproduces optimal power is more narrow and limited than that of the rotorsystem. In order to overcome these shortcomings, multi-ratiotransmissions can be used to provide appropriate torque and speed to therotor shaft by engaging gears of different ratios.

Friction clutches disengage the torque transfer path within atransmission and enable the engagement of an alternate gear system witha different reduction ratio. In such cases, torque is transmitted fromthe engine to the output shaft via frictional coupling between clutchplates.

While common for automobiles, the disadvantages of relying on frictionto provide torque throughout speed transitions arc; a lack ofoverrunning capability, increased debris generation, heat generationduring engagement, installed weight (also referred to as power density),increased potential for drive train shock loads during engagement anddisengagement, and potential for inadvertent disengagement while underload. Specifically, the need for additional components for overrunningcapability necessary for aircraft autorotation makes the overalltransmission assembly more complex and heavier.

From the foregoing, there is a need for a light-weight transmissionshifting method and mechanism overcoming the above-describedshortcomings of current fixed ratio and friction clutch basedmulti-ratio rotorcraft transmissions.

SUMMARY

A multi-ratio rotorcraft drive system and a method of changing gearratios thereof are disclosed. According to one embodiment, themulti-ratio rotorcraft drive system comprises a rotor system comprisingone or more rotors and one or more engines. Each engine of the one ormore engines is coupled to the rotor system through a multi-ratiotransmission. The multi-ratio transmission comprises an output shaftcoupled to the rotor system, an input shaft coupled to a respectiveengine of the one or more engines, a high speed clutch integrated into ahigh speed gear train, and a low speed clutch integrated into a lowspeed gear train. The high speed clutch and the low speed clutch arefreewheeling clutches without a friction plate and are capable ofdisconnecting the output shaft and the input shaft in an overrunningcondition when the output shaft spins faster than the input shaft. Themulti-ratio transmission shifts between the high speed gear train andthe low speed gear train by engaging or disengaging the high speedclutch and modulating speed of the coupled engine.

In one embodiment, the speed of a rotor system is reduced from a highspeed to a low speed. The speed of the rotor system is lowered from thehigh speed to a transitional speed. A first multi-ratio transmissioncoupled to a first engine is shifted from a high gear to a low gearwhile the speed of the rotor system is maintained at or near thetransitional speed. A second multi-ratio transmission coupled to asecond engine is shifted from a high gear to a low gear while the speedof the rotor system is maintained at or near the transitional speed. Thespeed of the first engine and second engine is restored to an optimumengine speed, causing the speed of the rotor system to be reduced to thelow speed.

In another embodiment, the speed of the rotor system is increased from alow speed to a high speed. First, the speed of the rotor system isincreased from the low speed to a transitional speed. The firstmulti-ratio transmission coupled to the first engine is shifted from alow gear to a high gear while the speed of the rotor system ismaintained at or near the transitional speed. The second multi-ratiotransmission coupled to the second engine is shifted from a low gear toa high gear while the speed of the rotor system is maintained at or nearthe transitional speed. The speed of the first engine and second engineis restored to an optimum engine speed, causing the speed of the rotorsystem to be increased to the high speed.

In accordance with the purpose of the various embodiments describedherein, as broadly described herein, the subject matter of this patentrelates to powered lift rotorcraft systems with a multi-ratiotransmission.

The above and other preferred features, including various novel detailsof implementation and combination of elements will now be moreparticularly described with reference to the accompanying drawings andpointed out in the claims. It will be understood that the particularmethods and apparatus are shown by way of illustration only and not aslimitations. As will be understood by those skilled in the art, theprinciples and features explained herein may be employed in various andnumerous embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included as part of the presentspecification, illustrate the presently preferred embodiment of thepresent invention and together with the general description given aboveand the detailed description of the preferred embodiment given belowserve to explain and teach the principles of the present invention.

FIG. 1 illustrates an exemplary high speed operation of a multi-rotor,multi-engine rotorcraft system with multi-ratio transmissions, accordingto one embodiment;

FIG. 2 illustrates an exemplary low speed operation of a multi-rotor,multi-engine rotorcraft system with multi-ratio transmissions, accordingto one embodiment;

FIG. 3 illustrates an exemplary block diagram for high speed to lowspeed transition, according to one embodiment;

FIG. 4 illustrates an exemplary block diagram for low speed to highspeed transition, according to one embodiment;

FIG. 5 illustrates an exemplary engine and rotor speed profile duringshift to low speed and back to high speed, according to one embodiment;

FIG. 6 illustrates an exemplary high speed configuration of multi-ratiorotorcraft drive system, according to one embodiment; and

FIG. 7 illustrates an exemplary low speed configuration of multi-ratiorotorcraft drive system, according to one embodiment;

It should be noted that the figures are not necessarily drawn to scaleand that elements of similar structures or functions are generallyrepresented by like reference numerals for illustrative purposesthroughout the figures. It also should be noted that the figures areonly intended to facilitate the description of the various embodimentsdescribed herein. The figures do not describe every aspect of theteachings described herein and do not limit the scope of the claims.

DETAILED DESCRIPTION

A multi-ratio rotorcraft drive system and a method of changing gearratios thereof are disclosed. According to one embodiment, themulti-ratio rotorcraft drive system comprises a rotor system comprisingone or more rotors and one or more engines. Each engine of the one ormore engines is coupled to the rotor system through a multi-ratiotransmission. The multi-ratio transmission comprises an output shaftcoupled to the rotor system, an input shaft coupled to a respectiveengine of the one or more engines, a high speed clutch integrated into ahigh speed gear train, and a low speed clutch integrated into a lowspeed gear train. The high speed clutch and the low speed clutch arefreewheeling clutches without a friction plate and are capable ofdisconnecting the output shaft and the input shaft in an overrunningcondition when the output shaft spins faster than the input shaft. Themulti-ratio transmission shifts between the high speed gear train andthe low speed gear train by engaging or disengaging the high speedclutch and modulating speed of the coupled engine.

In one embodiment, the speed of a rotor system is reduced from a highspeed to a low speed. The speed of the rotor system is lowered from thehigh speed to a transitional speed. A first multi-ratio transmissioncoupled to a first engine is shifted from a high gear to a low gearwhile the speed of the rotor system is maintained at or near thetransitional speed. A second multi-ratio transmission coupled to asecond engine is shifted from a high gear to a low gear while the speedof the rotor system is maintained at or near the transitional speed. Thespeed of the first engine and second engine is restored to an optimumengine speed, causing the speed of the rotor system to be reduced to thelow speed.

In another embodiment, the speed of the rotor system is increased from alow speed to a high speed. First, the speed of the rotor system isincreased from the low speed to a transitional speed. The firstmulti-ratio transmission coupled to the first engine is shifted from alow gear to a high gear while the speed of the rotor system ismaintained at or near the transitional speed. The second multi-ratiotransmission coupled to the second engine is shifted from a low gear toa high gear while the speed of the rotor system is maintained at or nearthe transitional. speed. The speed of the first engine and second engineis restored to an optimum engine speed, causing the speed of the rotorsystem to be increased to the high speed.

Each of the additional features and teachings disclosed herein can beutilized separately or in conjunction with other features and teachingsto provide an improved design for a multi-ratio rotorcraft drive system.Representative examples utilizing many of these additional features andteachings, both separately and in combination, are described in furtherdetail with reference to the attached drawings. This detaileddescription is merely intended to teach a person of skill in the artfurther details for practicing preferred aspects of the presentteachings and is not intended to limit the scope of the claims.Therefore, combinations of features disclosed in the following detaileddescription may not be necessary to practice the teachings in thebroadest sense, and are instead taught merely to describe particularlyrepresentative examples of the present teachings.

Moreover, the various features of the representative examples and thedependent claims may be combined in ways that are not specifically andexplicitly enumerated in order to provide additional useful embodimentsof the present teachings. In addition, it is expressly noted that allfeatures disclosed in the description and/or the claims are intended tobe disclosed separately and independently from each other for thepurpose of original disclosure, as well as for the purpose ofrestricting the claimed subject matter independent of the compositionsof the features in the embodiments and/or the claims. It is alsoexpressly noted that all value ranges or indications of groups ofentities disclose every possible intermediate value or intermediateentity for the purpose of original disclosure, as well as for thepurpose of restricting the claimed subject matter. It is also expresslynoted that the dimensions and the shapes of the components shown in thefigures are designed to help to understand how the present teachings arepracticed, but not intended to limit the dimensions and the shapes shownin the examples.

The present system and method enables the operation of a rotor system atmultiple speeds for a single engine speed using a multi-ratio drivesystem. The engine speed is maintained in its most efficient range whilevarying the rotor speed for optimum aircraft performance. The presentsystem and method provides a weight-efficient and power-efficientconfiguration without using friction clutches. In cases that an aircrafthas two engines, the present system and method allows the aircraft tohave power from at least one engine during normal shifting to facilitatethe change in rotor speed. Transmission ratios can be changed in flightwhile providing power from at least one engine to the rotor system atall times.

FIG. 1 illustrates an exemplary high speed operation of a rotorcraftsystem with multi-ratio transmissions, according to one embodiment.Rotorcraft system 100 has two engines 101 a and 101 b and respectivetransmissions 105 a and 105 b, which drive rotor system 110. Rotorsystem 110 has two rotors connected together via interconnect driveshaftsystem 120. In an alternative embodiment, rotor system 110 may becoupled to transmissions 105 a and 105 b through gears.

According to one embodiment, high speed clutch 102 and low speed clutch103 are sprag clutches. Sprag clutches are ‘one-way’ freewheelingclutches designed to transmit torque unidirectionally from the input(e.g., engine 101) to the output (e.g., rotor system 110). Due to their‘one-way’ design, sprag clutches transmit insignificant or no torqueduring an overrunning condition when the output shaft is spinning fasterthan the input shaft. Sprag clutches prevent a rotor from driving anengine should the engine lose power. Gear shifting of high speed clutch102 is achieved by modulating the speed of engine 101 and engaging ordisengaging high speed clutches 102.

According to one embodiment, high speed clutches 102 and low speedclutches 103 are self-energizing clutches that do not have frictionplates. A self-energizing clutch allows more efficient torque transferas the input and output shafts mate tighter. When the speed of the inputand output shafts are dissimilar in speed, the self-energizing clutchallows no torque transfer. With its internal driving mechanism, theself-energizing clutch automatically energizes and efficiently transmitstorque when an overrunning condition is removed.

Conventional self-energizing clutches do not have the capability tocontrol and modulate the speed of the output shaft with respect to thespeed of the input shaft without a proper clutch mechanism. For thisreason, a friction clutch may be used in series with a self-energizingfreewheeling clutch to provide such capability. However, frictionclutches require large friction surfaces to transmit torque from theinput shaft to the output shaft, which adds more weight to the mechanismand increases maintenance requirements due to the wear and tear ofengaging components. The present system and method eliminates the use offriction clutches and achieves significant advantages on efficiency,size, and weight over prior art rotorcraft drive clutch designs.

During high speed operation, both high speed clutch 102 and low speedclutch 103 are engaged. However, torque is transmitted only through highspeed clutch 102 from engine 101 to rotor 110 because low speed clutch103 is a one-way clutch in an overrunning condition where its outputspins faster than its input.

According to one embodiment, engine 101 runs at a cruising speed that isslower than the maximum speed where maximum operational efficiency ofengine 101 is achieved. For example, during cruise speed operation,engines 101 and rotors 110 run at 84% of their maximum speeds. Thepercentage of optimal speeds is selected for illustrative purposes only,and it is appreciated that any percentage may be, used Without deviatingfrom the scope of the present subject matter.

FIG. 2 illustrates an exemplary low speed operation of a rotorcraftsystem with multi-ratio transmissions, according to one embodiment. Forlow rotor speed operation, high speed clutch 102 is disengaged, and lowspeed clutch 103 transmits torque from engine 101 to rotor 110.According to one embodiment, low speed clutch 103 is permanently engagedfor both high and low speed operation so that gear shifting from thehigh gear ratio to the low gear ratio occurs by simply disengaging highspeed clutch 102. Conversely, shifting from the low gear ratio to highgear ratio occurs by re-engaging high speed clutch 102. After shiftingoccurs from the high gear to the low gear, engines 101 still run attheir optimal speed (e.g., 84% of their maximum speeds) while rotors 110spin at a low speed (e.g., 60% of its maximum speed). The transition ofrotor speed (e.g., 84% to 60%) occurs in several steps, which aredescribed below in greater detail.

For the purpose of illustration, the maximum engine speed and maximumrotor speed are symbolized as. E and R. In high gear, the maximum enginespeed E generates maximum rotor speed R. The rotor speed is calculatedby the following equation:

ω_(rotor)=r*ω _(engine)

where r is the gear ratio. In high gear, r_(high)=R/E, andr_(low)=ƒ*r_(high), in low gear, where ƒ is a gear reduction factor. Inview of the present example, the optimum engine speed is 0.84 E (84% ofthe maximum engine speed) that corresponds to the rotor speed 0.84 R(84% of maximum rotor speed) when engaged in the high speed gear.

FIG. 3 illustrates an exemplary block diagram for high speed to lowspeed transition, according to one embodiment. In the present example,the gear reduction factor, ƒ=0.714 is used, however it is appreciatedthat any other gear reduction factor might be used without deviatingfrom the scope of the present subject matter. The engine speeds of bothengines 101 a and 101 b are lowered from cruising speed (0.84 R) to aslower speed, for example, 0.714 R in the high speed gear such thatrotor system 110 spins at 0.714 R. When the engine speed of engine 101 bis reduced to just below 0.714 R, high speed clutch 102 b is in anoverrunning condition and is easily disengaged. Transmission 105 b isnow in the low gear. The torque path is switched from high speed clutch102 b to low speed clutch 103 b, although torque is not present orinsignificant when output shaft speed of clutch 103 b is greater thanits input shaft speed. After the gear shift, the engine speed of engine101 b is increased to 1.0 E to match the speed of the rotor system 110at 0.714 R (r_(high)* 0.714 E).

With engine 101 b running at full speed (1.0 E) and rotor system 110running at 0.714 R, similar shifting is performed on engine 101 a andtransmission 105 a to change the torque path from high speed clutch 102a to low speed clutch 103 a. After reducing the engine speed of engine101 a just below 0.714 R, high speed clutch 102 a is disengaged, andtransmission 105 a is in the low gear. After the gearshift, the enginespeed of engine 101 a is ramped up to 1.0 E so that both engines 101 aand 101 b are driving rotor system 110 at a speed of 0.714 R. After thegear shifting on both engines 101 a and 10 lb has occurred, low speedclutches 103 a and 103 b solely transmit torque to rotor system 110. Thespeeds of both engines 101 a and 101 b are lowered to their cruisingspeed at 0.84 E such that the speed of the rotor system is lowered tothe low speed, 0.60 R (r_(low)* 0.84 E).

According to one embodiment, transmissions 105 b and 105 a switch gearssequentially such that there is at least one engine powering rotorsystem 110 at all times. As shown in the previous example of high speedto low speed transition, gear switching may occur in sequence, but someintermediate steps for changing the ratio between engines 101 a and 101b and rotor system 110 might vary. For example, the engine speed ofengine 101 a may remain at 1.0 E while transmission 105 b shifts, orrotor system 110 may be freewheeling while gear shifting occurs. It isappreciated that the steps of speed adjustment and gear shifting mayoccur in different orders without deviating from the scope of thepresent subject matter.

According to one embodiment, a single transmission, engine, and rotorsystem can change ratios by controlling the rotor speed using the rotorcontrols as opposed to another engine and interconnect system. Thismethod permits the rotor system speed to maintain speed near thetransitional speed while the engine reduces speed and the clutchoverruns and disengages allowing a shift from high speed to low speed.Alternatively, to shift from a low speed to a high speed, the rotorsystem may be controlled to increase rotor speed taking advantage of therotorcraft altitude and speed, and enable the transmission to shift fromthe low speed to the high speed in the same manner as described for thehigh speed to low speed transition.

FIG. 4 illustrates an exemplary block diagram for low speed to highspeed transition, according to one embodiment. The same gear reductionratio, ƒ=0.714 is used in the present example to illustrate the speedtransition from low speed to high speed. Engine 101 runs at cruisingspeed (0.84 E), and rotor system 110 spins at a low speed, 0.60 R(r_(low)* 0.84 E) in its low gear. The speed of engines 101 a and 101 bis increased from the cruising speed (0.84 E) to the maximum speed (1.0E) to ramp up the rotor speed to the shifting speed, for example, 0.714R. The engine speed of engine 101 a is reduced to just below 0.714 E,causing high speed clutch 102 a to be in an overrunning condition. Highspeed clutch 102 a is engaged in the overrunning condition, whichchanges the torque path from low speed clutch 103 a to high speed clutch102 a when the speed of the output shaft matches the speed of the inputshaft. The engine speed of engine 101 a is changed to 0.714 E such thattorque is applied to rotor system 110 from engine 101 a.

With engine 101 a running at 0.714 E, transmission 105 a in high gear,and rotor system 110 running at 0.714 R, gear shifting is performed ontransmission 105 b and engine 101 b. The engine speed of engine 101 b isreduced to just below the transition speed (0.714 E), and high speedclutch 102 b is engaged. After the gear shifting, engine 101 b runs upto 0.714 E, and matches the rotor system at 0.714 R. After the shiftingis completed for both transmissions 105 a and 105 b, the speed of bothengines 101 a and 101 b is increased to their cruising speed at 0.84 Esuch that speed of rotor system 110 is increased to the cruising speed,0.84 R.

FIG. 5 illustrates an exemplary engine and rotor speed profile duringshift to low speed and back to high speed, according to one embodimentRotor speed 505 is reduced from a cruising speed, for example, 0.84 R toa slow speed, for example, 0.60 R, and ramped up back to the cruisingspeed, 0.84 R. During the speed transition of rotor system 110, theengine speeds 501 and 502 of engine 101 a and 101 b are illustrated asshown in FIG. 5. Transmission 105 a shifts gears from its high gear tolow gear while engine 101 a runs just below the speed of the rotors. Assoon as the gear shifting at transmission 105 a is completed,transmission 105 b shifts gears from its high gear to low gear in asimilar manner as transmission 105 a did. Gear shifting from low speedto high speed gears occurs in opposite orders as illustrated in thesecond half of the plot in FIG. 5.

FIG. 6 illustrates an exemplary high speed configuration of multi-ratiorotorcraft drive system, according to one embodiment. In high speedconfiguration, torque from engine 101 is transmitted via torque path 601through high speed clutch 102 to rotor 110. Low speed clutch 103 isalways engaged such that when high speed clutch 102 disengages, thetorque path from engine 101 to rotor 101 is transmitted through lowspeed clutch 103.

FIG. 7 illustrates an exemplary low speed configuration of multi- ratiorotorcraft drive system, according to one embodiment. In low speedconfiguration, high speed clutch 102 is disengaged such that torque istransmitted via torque path 701 through the output shaft of engine 101and low speed clutch 103 to rotor 110.

According to one embodiment, high speed clutch 102 and low speed clutch103 are overrunning clutches. When the output shaft is spinning fasterthan the input shaft of overrunning clutches, there is no torquetransmission. On the other hand, when the output shaft is not spinningfaster than the input shaft of overrunning clutches, the input andoutput shafts are instantaneously coupled, and torque is transmitted asif the input and output shafts are coupled without differentialrotation. This safety feature of overrunning clutches is especiallyuseful in the event of engine failure so that rotors can freely rotateby automatically disconnecting the engine.

According to one embodiment, the engine speed of engines 101 ismaintained in their most efficient range except during the speedtransition while varying the rotor speed for optimum performance.Although the two-ratio version is shown in the previous examples, adifferent number of clutch systems may be used in a drive system withmore than two gear ratios. The number of speeds may be determined byaircraft weight and performance requirements.

A multi-ratio rotorcraft drive system and a method of changing gearratios thereof have been described with respect to specific example andsubsystems. It will be apparent to those of ordinary skill in the artthat it is not limited to these specific examples or subsystems butextends to other embodiments as well.

We claim:
 1. A rotorcraft drive system comprising: a rotor systemcomprising one or more rotors; one or more engines, each engine of theone or more engines being coupled to the rotor system through amulti-ratio transmission, the multi-ratio transmission comprising: anoutput shaft coupled to the rotor system; an input shaft coupled to arespective engine of the one or more engines; a high speed clutchintegrated into a high speed gear train; and a low speed clutchintegrated into a low speed gear train, wherein the high speed clutchand the low speed clutch are freewheeling clutches without a frictionplate and are capable of disconnecting the output shaft and the inputshaft in an overrunning condition when the output shaft spins fasterthan the input shaft; and wherein the multi-ratio transmission shiftsbetween the high speed gear train and the low speed gear train byengaging or disengaging the high speed clutch and modulating speed ofthe coupled engine.
 2. The rotorcraft drive system of claim 1, whereinthe low speed clutch is permanently engaged.
 3. The rotorcraft drivesystem of claim 1, wherein the output shaft of the multi-ratiotransmission is coupled to the rotor system either through gears or aninterconnect driveshaft system
 4. The rotorcraft drive system of claim1, wherein the high speed clutch is selectively disengaged and engagedfor respective lowering and increasing the speed of the rotor system. 5.The rotorcraft drive system of claim 1, wherein the high speed clutchand the low speed clutch are self-energizing freewheeling clutches withan overrunning capability.
 6. The rotorcraft drive system of claim 1,where the rotor system does not transmit torque to the one or moreengines via the high speed clutch or the low speed clutch in theoverrunning condition.
 7. A method of reducing the speed of a rotorsystem driven by one or more engines from a high speed to a low speed,the method comprising: lowering the speed of the rotor system from thehigh speed to a transitional speed; shifting a first multi-ratiotransmission coupled to a first engine of the one or more engines from ahigh gear train integrating a high speed clutch to a low gear trainintegrating a low speed clutch while the speed of the rotor system ismaintained at or near the transitional speed; shifting a secondmulti-ratio transmission coupled to a second engine of the one or moreengines from a high gear train integrating a high speed clutch to a lowgear train integrating a low speed clutch while the speed of the rotorsystem is maintained at or near the transitional speed; and restoringthe speed of the first engine and second engine to an optimum enginespeed, causing the speed of the rotor system to be reduced to the lowspeed.
 8. The method of claim 7, wherein the lowering the speed of therotor system from the high speed to a transitional speed furthercomprising: lowering the speed of the first engine just below thetransitional speed of the rotor system; disengaging the high speedclutch of the first multi-ratio transmission; and ramping the speed ofthe first engine to match the transitional speed of the rotor system. 9.The method of claim 7, wherein the lowering the speed of the rotorsystem from the high speed to a transitional speed further comprising:lowering the speed of the second engine just below the transitionalspeed of the rotor system; disengaging the high speed clutch of thesecond multi-ratio transmission; and ramping the speed of the secondengine to the transitional speed of the rotor system.
 10. The method ofclaim 7, wherein the low speed clutch of the first multi-ratiotransmission and the low speed clutch of second multi-ratio transmissionare permanently engaged.
 11. The method of claim 7, wherein the highspeed clutch of the first multi-ratio transmission and the high speedclutch of second multi-ratio transmission are selectively disengaged andengaged for respective lowering and increasing the speed of the rotorsystem.
 12. The method of claim 7, wherein the high speed clutch and thelow speed clutch of the first multi-ratio transmission and secondmulti-ratio transmission are self-energizing freewheeling clutches withan overrunning capability.
 13. A method of increasing the speed of arotor system driven by one or more engines from a low speed to a highspeed, the method comprising: increasing the speed of the rotor systemfrom the low speed to a transitional speed; shilling a first multi-ratiotransmission coupled to a first engine of the one or more engines from alow gear train integrating a low speed clutch to a high gear trainintegrating a high speed clutch while the speed of the rotor system ismaintained at or near the transitional speed; shifting a secondmulti-ratio transmission coupled to a second engine of the one or moreengines from a low gear train integrating a low speed clutch to a highgear train integrating a high speed clutch while the speed of the rotorsystem is maintained at or near the transitional speed; and restoringthe speed of the first engine and second engine to an optimum enginespeed, causing the speed of the rotor system to be increased to the highspeed.
 14. The method of claim 13, wherein the increasing the speed ofthe rotor system from the low speed to a transitional speed furthercomprising: lowering the speed of the first engine just below thetransitional speed of the rotor system; engaging the high speed clutchof the first multi-ratio transmission; and ramping the speed of thefirst engine to match the transitional speed of the rotor system. 15.The method of claim 13, wherein the increasing the speed of the rotorsystem from the low speed to a transitional speed further comprising:lowering the speed of the second engine just below the transitionalspeed of the rotor system; engaging the high speed clutch of the secondmulti-ratio transmission; and ramping the speed of the second engine tothe transitional speed of the rotor system.
 16. The method of claim 13,wherein the low speed clutch of the first multi-ratio transmission andthe low speed clutch of second multi-ratio transmission are permanentlyengaged.
 17. The method of claim 13, wherein the high speed clutch ofthe first multi-ratio transmission and the high speed clutch of secondmulti-ratio transmission are selectively disengaged and engaged forrespective lowering and increasing the speed of the rotor system. 18.The method of claim 13, wherein the high speed clutch and the low speedclutch of the first multi-ratio transmission and second multi-ratiotransmission are self-energizing freewheeling clutches with anoverrunning capability.